1. Field of the Invention
The present invention relates to optical gyroscopes and more particularly to passive ring resonator gyroscopes; these have bias frequency errors resulting from mechanically or thermally induced dimensional changes that influence tuning.
2. Description of Prior Art
In a passive ring resonator gyroscope, a pair of monochromatic light beams counterpropagate about closed-loop optical paths, where the paths form a resonator. The stability of the path length between reflective surfaces forming the closed path is critical in maintaining resonance in the passive ring resonator cavity since dimensional changes contribute to bias frequency errors. The relationship between a linear laser.sup.1 and a ring resonator to form a prior art passive ring resonator gyro is depicted in an article by S. EZEKIEL and S. R. BALSAMO titled "A Passive Ring Laser Gyroscope", Applied Physics Letters, Vol. 30, No. 9, May 1, 1977, pg. 478-480. (NOTE: Usually a resonator is conceived as a linear or standing wave resonator in which the light completes an optical round trip by reflecting off a mirror and retracing its path. These forward and backward waves create a standing wave in the cavity. In a ring resonator, the light completes an optical round trip without retracing its path and hence the path encloses an area as shown in Ezekiel's paper.) FNT .sup.1 For description of lasers and resonators refer to: Yariv, A., QUANTUM ELECTRONICS (John Wiley & Sons, 1975) or Sargent, M., et. al., LASER PHYSICS (Addison-Wesley Pub., 1974).
In the passive ring resonator, such as that described in the EZEKIEL reference, the two beams, traveling in opposite directions around the closed-loop optical path, are injected into the passive ring resonator from a single frequency light source. As the ring resonator gyroscope cavity rotates in inertial space, the two counterpropagating beams travel unequal path lengths. This path difference, due to rotation in inertial space, gives rise to a relative frequency difference (Sagnac effect.sup.2) between the two counterpropagating beams. (NOTE: A ring resonator, as opposed to a linear resonator, can exhibit the Sagnac effect and detect inertial rotation.) FNT .sup.2 E. J. Post, "Sagnac Effect", Review of Modern Physics, Vol. 39, No. 2, April 1967, p. 475-493.
The relative frequency difference is detected as an electrical beat signal which is then electronically interpreted to indicate the direction and inertial rate of rotation of the passive gyro about the gyro's sensitive axis. The sensitive axis of the gyro is along the direction normal to the plane of the passive resonator.
The single frequency light source for the passive resonator is typically an external linear laser. Spectra Physics Inc. of Sunnyvale, Calif. produces stabilized lasers with the required characteristics.
It is known that bias errors in the detected signal of a ring resonator gyro result from dimensional changes in the laser and in the passive ring resonator. Bias errors also result from Fresnel Drag; these errors arise from the presence of gases (e.g. air) in the path of the counterpropagating beams in the resonator. Bias errors are typically characterized as a frequency difference between the two beams which is not related to the rotation rate. Bias errors are sometimes detected as a frequency difference in the absence of rotation or as post calibration changes in the frequency difference for a specific absolute inertial rotation rate.
The Passive Ring Resonator Gyroscope of the type described in the EZEKIEL reference is typically constructed by placing optical elements, such as mirrors, beamsplitters, etc. on an optical bench. The location, spacing and geometrical relationships between the elements of the gyro function to enhance the passive ring resonator gyroscope's sensitivity and stability. Experimental passive ring resonator gyroscopes typically have path lengths of a few meters making them unsuitable for use as a navigational instrument. The large size of prior art passive ring resonator gyroscopes, such as that characterized in the EZEKIEL reference, also contributes to the likelihood of bias errors due to mechanical coupling and mechanical drift of the optical elements in response to physical and thermal forces acting on the laser and on the optical table or bench.